A "Sense FET" is an example of the type of transistor with which this invention is most advantageously used. Such a transistor typically includes at least two transistors connected in parallel within the same package, with a common drain electrode, a common gate electrode, and two source electrodes. One source electrode usually carries a relatively high level of load current, while the other source electrode (referred to herein as the sense electrode) carries a relatively lower level of current that is a known fraction of the load current. By sensing the current carried by the sense electrode, one can determine the level of the load current.
Sensing the current carried by the sense electrode is conventionally accomplished by placing a resistor (the sense resistor) in series with the sense electrode, and measuring the voltage drop across the sense resistor. While this technique is adequate for some applications, it is not entirely satisfactory for situations where higher accuracy is desired. The lack of accuracy in the conventional technique arises because
AP01968 the voltage drop across the sense resistor results in unequal gate-to-source voltages between the transistor carrying the sense current and the transistor carrying the load current. If the inequality between the gate-to-source voltages is more than about 300 millivolts, the sense current will be more or less than the desired percentage of the load current.